Alkali-metal-embedded in monolayer MoS<Subscript>2</Subscript>: optical properties and work functions

Embedding alkali-metal in monolayer MoS₂ has been investigated by using first principles with density functional theory. The calculation of the electronic and optical properties indicates that alkali-metal was embedded in monolayer MoS₂ appearing almost metallic behavior, and the MoS₂ layer shows clear p-type doping behavior. The covalent bonding appears between the alkali-metal atoms and defective MoS₂. More importantly, embedding alkali-metal can increase the work function for monolayer MoS₂. Furthermore, the absorption spectrum of monolayer MoS₂ is red shifted because of alkali metal embedding. Accordingly, this study will provide the theoretical basis for producing the alkali-metal-doped monolayer MoS₂ radiation shielding and photoelectric devices.     

The effect of carrier gas flow rate on the growth of MoS<Subscript>2</Subscript> nanoflakes prepared by thermal chemical vapor deposition

Molybdenum disulfide nanoflakes (MoS₂) are superior material for their semiconducting properties. For bulk and monolayer MoS₂ the band gap changes from indirect-to-direct, respectively. So, it exhibits promising prospects in the applications of optoelectronics and valleytronics, such as solar cells, transistors, photodetectors, etc. In this research, the influence of different Ar flow rates as the carrier gas, is investigated for growing MoS₂ nanoflakes on silicon substrates using one-step thermal chemical vapor deposition by simultaneously evaporating of solid sources like sulfur and molybdenum trioxide powders. The structural and optical properties of the obtained nanoflakes are assessed by using X-ray diffraction pattern, scanning electron microscopy, UV–visible absorption, photoluminescence and Raman spectroscopy. It is shown that, Ar gas flow rate is strongly affects on the final products as few-layer MoS₂ structures. Moreover, the abundance of MoS₂ in comparison to MoO₂ and MoO₃ structures, in the obtained nanoflakes, is influenced by the Ar flow rate.     

From layers to nanotubes: Transition metal disulfides TMS<Subscript>2</Subscript>

MoS₂ and WS₂ layered transition-metal dichalcogenides are indirect band gap semiconductors in their bulk forms. Thinned to a monolayer, they undergo a transition and become direct band gap materials. Layered structures of that kind can be folded to form nanotubes. We present here the electronic structure comparison between bulk, monolayered and tubular forms of transition metal disulfides using first-principle calculations. Our results show that armchair nanotubes remain indirect gap semiconductors, similar to the bulk system, while the zigzag nanotubes, like monolayers, are direct gap materials, what suggests interesting potential applications in optoelectronics.     

Chirality effect of mechanical and electronic properties of monolayer MoS2 with vacancies

We present first principles theory calculations about the chirality and vacancy effects of the mechanical and electronic properties of monolayer MoS₂. In the uni-axial tensile tests, chirality effect of the mechanical properties is negligible at zero strain and becomes significant with the increasing strain, regardless of vacancies. The existence of vacancies decreases the Young's modulus and ultimate strength of the MoS₂ structure. During the uni-axial tensile tests, the band gap decreases with the increasing strain, regardless of chirality and vacancies. The band gap is reduced with the intermediate state brought by the existence of vacancies. No chirality effect can be observed on the band gap variations of perfect MoS₂. Chirality effect appears to the band gap variation of defected MoS₂ due to the local lattice relaxation near the vacancies.     

Transient optical absorption and luminescence of lithium triborate LiB<Subscript>3</Subscript>O<Subscript>5</Subscript>

The transient optical absorption and luminescence of LiB₃O₅ (LBO) nonlinear crystals in the visible and UV spectral ranges were studied. Measurements made using absorption optical spectroscopy with nsscale time resolution revealed that the transient optical absorption (TOA) in LBO originates from optical transitions in hole centers and that the kinetics of optical density relaxation are rate-limited by interdefect nonradiative tunneling recombination involving these hole centers and the Li⁰ electronic centers, which represent neutral lithium atoms. At 290 K, the Li⁰ centers can migrate in a thermally stimulated, one-dimensional manner, a process which is not accompanied by carrier delocalization into the conduction or valence band. It is shown that the pulsed LBO cathodoluminescence kinetics is rate-limited by a recombination process involving two competing valence-band-mediated hole centers and shallow B²⁺ electronic centers. The radiative recombination accounts for the characteristic σ-polarized LBO luminescence in the 4.0-eV region.     

Giant Three‐Photon Absorption in Monolayer MoS2 and Its Application in Near‐Infrared Photodetection

For decades, there has been extensive research on exploring fundamental physical mechanisms for strong and fast optical nonlinearities. One of the important nonlinear‐optical mechanisms is multiphoton absorption which has a wide range of photonic applications. Herein, a theoretical model is proposed for three‐photon absorption (3PA) in monolayer MoS₂. The model shows that the 3PA coefficients are on the order of 0.1 cm³/GW². As compared to bulk semiconductors, these coefficients are enhanced by several orders of magnitude due to excitonic effects. Such exciton‐enhanced 3PA is validated by light‐intensity‐dependent photocurrent measurements on a monolayer MoS₂ photodetector with femtosecond laser pulses. These results lay both theoretical and experimental foundation for developing sensitive near‐infrared MoS₂‐based three‐photon detectors.     

Transport properties of MoS<Subscript>2</Subscript> nanoribbons: edge priority

We report about results from density functional based calculations on structural, electronic and transport properties of one-dimensional MoS₂ nanoribbons with different widths and passivation of their edges. The edge passivation influences the electronic and transport properties of the nanoribbons. This holds especially for nanoribbons with zigzag edges. Nearly independent from the passivation the armchair MoS₂ nanoribbons are semiconductors and their band gaps exhibit an almost constant value of 0.42 eV. Our results illustrate clearly the edge priority on the electronic properties of MoS₂ nanoribbons and indicate problems for doping of MoS₂ nanoribbons.     

Electronic and transport properties of heterophase compounds based on MoS<Subscript>2</Subscript>

New heterophase superlattices based on MoS₂ are studied in detail by the electron density functional theory. It is shown that the incorporation of the 1Т phase in the 2H-MoS₂ monolayer is responsible for the formation of electronic levels near the Fermi level and quantum wells in the transverse direction of superlattices. The proposed lateral heterophase structures of transition metal dichalcogenides are promising for the construction of new elements of nanoelectronics.     

Theoretical Study of the Structural,Electronic, Chemical Bonding and Optical Properties of the A2<Subscript>1</Subscript><Emphasis Type="Italic">am</Emphasis> Orthorhombic SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript>

The structural parameters, density of states, electronic band structure, charge density, and optical properties of orthorhombic SrBi₂Ta₂O₉ have been investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principle density functional theory (DFT). The calculated structural parameters were in agreement with the previous theoretical and experimental data. The band structure showed an indirect (S to Γ) band gap with 2.071 eV. The chemical bonding along with population analysis has been studied. The complex dielectric function, refractive index, and extinction coefficient were calculated to understand the optical properties of this compound, which showed an optical anisotropy in the components of polarization directions (100), (010), and (001).     

Transient hole-polaron optical absorption in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

Results of a study of transient optical absorption (TOA) and luminescence of lithium gadolinium orthoborate Li₆Gd(BO₃)₃ (LGBO) in the visible and UV spectral regions are presented. As revealed by absorption optical spectroscopy with nanosecond time resolution, the LGBO TOA derives from optical transitions in hole centers, with the optical density relaxation kinetics being mediated by interdefect tunneling recombination involving these centers and neutral lithium atoms acting as electronic Li⁰ centers. At 290 K, the Li⁰ centers are involved in thermostimulated migration, which is not accompanied by carrier transfer to the conduction or valence band. The slow components of the TOA decay kinetics, with characteristic times ranging from a few milliseconds to seconds, have been assigned to diffusion-limited annihilation of lithium interstitials with vacancies. The mechanisms responsible for the creation and relaxation of short-lived Frenkel defect pairs in the LGBO cation sublattice have been analyzed.     

Electronic structures,magnetic properties and band alignments of 3d transition metal atoms doped monolayer MoS2

Utilizing first-principles calculations, the electronic structures, magnetic properties and band alignments of monolayer MoS₂ doped by 3d transition metal atoms have been investigated. It is found that in V, Cr, Mn, Fe-doped monolayers, the nearest neighboring S atoms (S_NN) are antiferromagnetically polarized with the doped atoms. While in Co, Ni, Cu, Zn-doped systems, the S_NN are ferromagnetically coupled with the doped atoms. Moreover, the nearest neighboring Mo atoms also demonstrate spin polarization. Compared with pristine monolayer MoS₂, little change is found for the band edges' positions in the doped systems. The Fermi level is located in the spin-polarized impurity bands, implying a half-metallic state. These results provide fundamental insights for doped monolayer MoS₂ applying in spintronic, optoelectronic and electronic devices.     

Electronic Structure,Optical Properties,and Pressure Behavior of the CdB<Subscript>4</Subscript>O<Subscript>7</Subscript> and HgB<Subscript>4</Subscript>O<Subscript>7</Subscript> Compounds

Ab initio calculations of the structural, electronic, and optical properties of the CdB₄O₇ and HgB₄O₇ tetraborate compounds in three structural modifications with the Pbca, Cmcm, and Pmn2₁ symmetry have been performed in the framework of the density functional theory using the VASP package. The calculations of the electronic band structure showed that these compounds in all the investigated modifications are dielectrics with a band gap of 2–4 eV. The calculation of the structural properties of the tetraborates under pressure showed that the phase transition between the Pbca and Pmn2₁ structures in cadmium and mercury tetraborates occurs under pressures of 4.8 and 4.7 GPa, respectively.     

The interfacial properties of SrRuO<Subscript>3</Subscript>/MoS<Subscript>2</Subscript> heterojunction: a first-principles study

First-principles calculation was used to study the interfacial properties of theSrRuO₃ (1 1 1)/MoS₂(√3 × √3) heterojunction. It is found that the huge magneticmoments in of monolayer MoS₂ largely originate from the Ru-S hybridization for theRu-terminated interface. Moreover, for the SrO-terminated interface, we studied mainly themetal and semiconductor contact characteristic. The calculated results show that theSchottky barrier height can be significantly reduced to zero for the SrO-terminatedinterface. Schottky barrier heights dominate the transport behavior of theSrRuO₃/MoS₂ interface. Our results not only have potentialapplications in spintronics devices, but also are in favour of the scaling of field effecttransistors.     

Electronic structure,ferroelectricity and optical properties of CaBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript>

Using first-principles calculations based on density-functional theory in its local-density approximation, we investigated the Electronic structure, ferroelectricity and optical properties of CaBi₂Ta₂O₉ (CBT) for the first time. It is found that CBT compound has an indirect band gap of 3.114 eV and the O 2s and 2p states are strongly hybridized with the 6s states of Bi which belong to the (Bi₂O₂)²⁺ planes. The quite strong Ta–O and Bi–O hybridization is the primary source for ferroelectricity. Our results imply that the interaction between Bi and O is highly covalent. The anisotropy occurs mainly above 4 eV in the optical properties. The different optical properties have been discussed.     

Layer‐dependent resonant Raman scattering of a few layer MoS2

We report resonant Raman scattering of MoS₂ layers comprising of single, bi, four and seven layers, showing a strong dependence on the layer thickness. Indirect band gap MoS₂ in bulk becomes a direct band gap semiconductor in the monolayer form. New Raman modes are seen in the spectra of single‐ and few‐layer MoS₂ samples which are absent in the bulk. The Raman mode at ~230 cm⁻¹ appears for two, four and seven layers. This mode has been attributed to the longitudinal acoustic phonon branch at the M point (LA(M)) of the Brillouin zone. The mode at ~179 cm⁻¹ shows asymmetric character for a few‐layer sample. The asymmetry is explained by the dispersion of the LA(M) branch along the Γ‐M direction. The most intense spectral region near 455 cm⁻¹ shows a layer‐dependent variation of peak positions and relative intensities. The high energy region between 510 and 645 cm⁻¹ is marked by the appearance of prominent new Raman bands, varying in intensity with layer numbers. Resonant Raman spectroscopy thus serves as a promising non invasive technique to accurately estimate the thickness of MoS₂ layers down to a few atoms thick. Copyright © 2012 John Wiley & Sons, Ltd.     

Surface Plasmon Polaritons in MoS<Subscript>2</Subscript> Nanostructures

The surface plasmon polaritons (SPPs) in monolayer MoS₂ nanostructures are theoretically investigated in detail. Our study shows that the strong SPPs are induced in gigahertz (GHz) frequency range. The frequencies of SPPs are very sensitive on the substrates in the nanostructures. Moreover, the frequency of such SPPs can be controlled by varying the electron densities. Our study can be applied to understand the recent experimental results and is relevant to the applications of plasmonic nano-devices based on MoS₂.     

Cr和W掺杂的单层MoS2电子结构的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波赝势方法, 研究了Cr和W掺杂对单层二硫化钼(MoS₂)晶体的电子结构性质的影响. 计算结果表明: 当掺杂浓度较高时, W对MoS₂的能带结构几乎没有影响, 而Cr的掺杂则影响很大, 表现为能带由直接带隙变为间接带隙, 且禁带宽度减小. 通过进一步分析, 得出应力的产生是导致Cr掺杂的MoS₂电子结构变化的最直接的原因.     

A study on monolayer MoS2 doping at the S site via the first principle calculations

Through the first principle calculation, electronic properties of monolayer MoS₂ doped with single, double, triple and tetra-atoms of P, Cl, O, Se at the surface S site are discussed. Among the substitutional dopant, our calculation results show that when P atoms are doped on a monolayer MoS₂, a shift in the Fermi energy into the valence band is observed, making the system p-type. Meanwhile, band gap gradually decreases as increasing the number of P atoms. On the contrary, Cl is identified as a suitable n-type dopant. It is observed that Cl for initial three dopant behaved as magnetic and afterwards returned to non-magnetic behavior. The band gap of the Cl doped system is also dwindling gradually. Finally, O and Se doped systems have little effect on electronic properties near band gap. Such doping method at the S site, and the TDOS and PDOSs of each doping system provide a detailed of understanding toward working mechanism of the doped and the intrinsic semiconductors. This doping model opens up an avenue for further clarification in the doping systems as well as other dopant using this method.     

First-principles studies of the electronic structure and optical properties of AgBO<Subscript>3</Subscript> (B=Nb,Ta) in the paraelectric phase

The electronic energy-band structure, density of states (DOS), and optical properties of AgBO₃ in the paraelectric cubic phase have been studied by using density functional theory within the local density approximation for exchange-correlation for the first time. The band structure shows a band gap of 1.533 eV (AgNbO₃)and 1.537 eV (AgTaO₃)at (M-⌈)point in the Brillouin zone. The optical spectra of AgBO₃ in the photon energy range up to 30 eV are investigated under the scissor approximation. The real and imaginary parts of the dielectric function and — thus the optical constants such as reflectivity, absorption coefficient, electron energy-loss function, refractive index, and extinction coefficient — are calculated. We have also made some comparisons with related experimental and theoretical data that is available.      

Preparation and optical properties of nanocrystalline thin films in the ZnO-TiO<Subscript>2</Subscript> system

Nanocrystalline compound thin films of ZnO-TiO₂ with different Zn/Ti atomic ratios were prepared by radio frequency magnetron reactive sputtering. The optical constants and the optical band gap were investigated using spectroscopic ellipsometry and the optical absorption spectrum. It was found that the cubic ZnTiO₃ phase can be obtained with the atomic ratio of Zn to Ti of about 1:1, and transforms to rhombohedral ZnTiO₃ phase and a phase mixture of rhombohedral ZnTiO₃ and ZnO with increasing Zn content. The refractive index decreases with the increase of Zn content, and the extinction coefficient in the visible range is near zero. The optical band gap was derived from the modeling of ellipsometry data and extinction coefficient spectra, and compared with that obtained from optical absorption spectrum, and it was found that the optical band gaps obtained by these three methods are consistent with each other. PACS 42.70.-a; 68.55.Jk; 78.20.Ci; 81.15.Cd